Characterizing laser output in a HAMR device

ABSTRACT

At least one laser input current is applied to a laser in a heat assisted magnetic recording device. Laser output power of the laser is measured at the at least one applied laser current. A relationship is characterized amongst temperature, applied laser input current and laser output power. Laser current is set to an optimal laser current as determined at manufacturing. A metric of recording performance is measured to determine if the relationship is acceptable.

BACKGROUND

Heat-assisted magnetic recording (HAMR) is a data storage technique fordata storage devices where a small laser is used to heat the part of therecording medium that is being written. The heat changes the magneticproperties of the portion of the recording medium while the recordingmedium is being recorded, which reduces or removes the superparamagneticeffect while recording takes place so as to increase the amount of datathat can be held on the recording medium.

The discussion above is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter.

SUMMARY

A method of characterizing laser output in a heat assisted magneticrecording device can be performed by control circuitry in a data storagedevice. At least one laser input current is applied to a laser in a heatassisted magnetic recording device. Laser output power of the laser ismeasured at the at least one applied laser current. A relationship ischaracterized amongst temperature, applied laser input current and laseroutput power. Laser current is set to an optimal laser current asdetermined at manufacturing. A metric of recording performance ismeasured to determine if the relationship is acceptable.

In another embodiment, a method includes measuring a first laser outputpower of a laser used to record data on a head assisted magneticrecording medium at a first laser input current, measuring a secondlaser output power of the laser at a second laser input that is greaterthan or less than the first laser input current, characterizing arelationship amongst temperature, laser input current and laser outputpower and verifying the relationship by measuring a metric of recordingperformance.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. The claimed subject matter is not limited to implementationsthat solve any or all disadvantages noted in the background.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of exemplary components of a data storagedevice including a head stack assembly and a medium.

FIG. 2 is a schematic diagram of a side view of a head gimbal assembly(HGA).

FIG. 3 illustrates an enlarged diagram of a trailing end of a slider ofthe HGA illustrated in FIG. 2.

FIG. 4 illustrates a graphical representation illustrating therelationship of applied laser current to sensed laser output power atdifferent temperatures.

FIG. 5 illustrates a graphical representation illustrating therelationship of optimal applied laser current of a laser versus thetemperature of the data storage device.

FIG. 6 is a block diagram illustrating a method of calibrating laserdiode current in a HAMR device during normal device operation accordingto one embodiment.

FIG. 7 illustrates a re-characterization method that can be performedwhen a magnitude of power error determined in FIG. 6 is too large or canbe initiated for other reasons.

FIG. 8 is a block diagram illustrating a method of calibrating laserdiode current in a HAMR device during normal device operation accordingto another embodiment.

DETAILED DESCRIPTION

This disclosure describes characterizing laser output in a Heat AssistedMagnetic Recording (HAMR) device during normal device operation toimprove the quality of the recording signal. In other words, arelationship amongst temperature, applied laser input current and laseroutput power is characterized. After characterization, the relationshipis verified by measuring a metric of recording performance to determineif the relationship is acceptable.

FIG. 1 is a simplified block diagram of an exemplary data storage device100 that can be used in embodiments described herein. Data storagedevice 100 includes control circuitry 102, which is used for controllingoperations of data storage device 100 with the use of programming storedin memory 104. Control circuitry 102 may be coupled to a buffer 106through a read/write channel 110. Buffer 106 can temporarily store userdata during read and write operations and may include the capability oftemporarily storing access operations pending execution by controlcircuitry 102.

Data storage device 100 includes storage medium or magnetic recordingmedium (i.e., disc) 108 and a suspension 116 supporting a transducinghead 118 (in this case a HAMR transducing head) that can read and writedata to medium 108. In the embodiment illustrated in FIG. 1, the storagemedium 108 is illustrated as being a rotatable disc. Data storage device100 also includes a preamplifier (preamp) 107 for generating a writesignal applied to transducing head 118 during a write operation, and foramplifying a read signal emanating from transducing head 118 during aread operation. In some embodiments, preamp 107 also includescompensation circuitry 109. Compensation circuitry 109 will be discussedin detail below.

Control circuitry 102 executes read and write operations on data storagemedium 108. These read/write operations executed by control circuitry102 may be performed directly on data storage medium 108 or throughread/write channel 110. Read/write channel 110 receives data fromcontrol circuitry 102 during a write operation, and provides encodedwrite data to data storage medium 108 via preamp 107. During a readoperation, read/write channel 110 processes a read signal via preamp 107in order to detect and decode data recorded on data storage medium 108.The decoded data is provided to control circuitry 102 and ultimatelythrough an interface 112 to an external host 114.

External host 114 contains logic (e.g., a processor) capable of issuingcommands to data storage device 100. Although FIG. 1 illustratesexternal host 114 as being a single host, data storage device 100 can beconnected through interface 112 to multiple hosts. Via interface 112,data storage device 100 receives data and commands from external host114 and can provide data to external host 114 based on commands executedby control circuitry 102.

FIG. 2 illustrates an enlarged side view of a head gimbal assembly (HGA)120 illustrating a suspension 116 supporting a slider 122 by a gimbal124. Slider 122 includes transducing head 118, which is rotatablerelative to suspension 116 via gimbal 124. Transducing head 118 islocated at a trailing edge of slider 122 and is held proximate tosurface 109 of medium 108 for reading and writing data.

HAMR transducing heads, such as a transducing head 118, use an energysource to locally heat a small portion of a recording medium to overcomesuperparamagnetic effects that limit the areal data density of amagnetic medium, such as medium 108. The heating of the medium raises aregion of the medium's temperature above a set temperature, allowing forit to be magnetized by a magnetic writer. The medium quickly cools as itrotates away from the energy source and therefore magnetically freezesthe written pattern for stable, long-term storage of data.

FIG. 3 illustrates an enlarged diagram of a trailing end of slider 122.HAMR transducing head 118 may include optical components, such as anoptical wave guide 119, that direct, concentrate and transform lightenergy from a laser assembly 126 to heat medium 108. Laser assembly 126includes a laser diode that receives a current input and applies laserenergy onto medium 108 through optical wave guide 119. The HAMR mediumhot spot may need to be smaller than the diffraction limit of light. Oneway to achieve such small hot spots is to use an optical near fieldtransducer (NFT) 129.

Control of the applied laser energy in a HAMR device is essential toperformance. If the heat energy imparted to the medium 108 is too lowthen medium 108 is not sufficiently heated, and the recorded signal isof a poor quality. If the energy is too high, the recorded signal ofadjacent tracks may be partially erased, which causes degradation.Moreover, the energy can change even if the current of the heat energyis constant. For example, the laser energy for a given laser currentvaries with temperature and also varies with other effects, such as withlaser diode aging or other component aging. For example, as componentsage, the amount of applied laser current needed to achieve the samedegree of media heating may vary.

In one embodiment, laser diode input current may be controlled by aregister in preamplifier 107 (FIG. 1). Preamplifier 107 contains adigital-to-analog converter (DAC) to convert the programmed registervalue into an applied current. The laser energy output from transducinghead 118 (FIGS. 2 and 3) onto medium 108 can vary. Even if the currentto the laser diode is accurate and constant, the power output from thelaser diode may not. For example, a forward voltage drop of the laserdiode can cause this relationship to vary. In addition, thepreamplifier's applied current may not always be accurate and may alsovary. Temperature has a strong effect on all of these variations.

There are two parameters that are critical to drive quality—thebit-error-rate (BER) of the written track on the media and thedegradation imparted to adjacent tracks (adjacent track erasure or ATE)by the write operation. Changes in laser power impact both of theseparameters. Unfortunately, to perform BER and ATE measurements well,many revolutions of the media are required. In addition, experimentallyperforming these measurements may cause degradation to the data onadjacent tracks. Therefore, performing BER and ATE measurements are notpractical to perform on a frequent basis while the drive is in normaloperation.

Two parameters that can be sensed regularly without performancedegradation include temperature and laser output power. Temperature canbe sensed periodically using a thermistor 128, for example. Laser outputpower can also be sensed in real-time, for example, with a sensor suchas a photodiode 127 or bolometer 131. In the embodiment illustrated inFIG. 3, photodiode 127 is part of laser assembly 126, which can bemanufactured on each transducing head and can be used to measure thelaser power or energy within the recording head. The arrow withintransducing head 118 in FIG. 3 illustrates the path of laser energythrough optical wave guide 119 from laser assembly 126 to NFT 129. Asshown, laser energy emanates from laser assembly 126 and energy from NFT129 heats a portion of medium 108. In an alternative embodiment,bolometer 131 is coupled to optical wave guide 119 and may also measurelaser output power in recording head 118.

There are three general modes of operation for the laser diode in a HAMRdrive. When idle, the diode is fully off or inactive (no appliedcurrent). When writing data, the diode is fully on or active with anapplied current sufficient to record or erase data to medium 108. Inpreparation for writing, the laser diode is partially on or biased witha current insufficient to record or erase data to medium 108.

FIG. 4 illustrates a graphical representation 240 illustrating therelationship of applied laser current or laser diode (LD) current (onthe x-axis) to sensed laser output power (on the y-axis) at differenttemperatures as measured during the engineering phase. The sensed laseroutput power is measured by photodetector 127 or bolometer 131 and istypically measured in terms of sensor voltage or current. Photodetector127 converts photons to electrons, which in turn lead to a voltage thatcan be measured by preamp 107. Bolometer 131 measures the power ofincident electromagnetic radiation via the heating of the material ofoptical wave guide 119 with a temperature-dependent electricalresistance. As illustrated by graphical representation 240, therelationship can be, but not limited to, linear, and therefore can bedescribed by equation(s) or tables that model such a relationship. Forexample, if the relationship is linear, the following equations can beused:PD Voltage=m[temp]×LaserCurrent+b[temp]  (Eqn. 1)

$\begin{matrix}{{{Laser}\mspace{14mu}{Current}} = \left( \frac{{PDVoltage} - {b\lbrack{temp}\rbrack}}{m\lbrack{temp}\rbrack} \right)} & \left( {{Eqn}.\mspace{14mu} 2} \right)\end{matrix}$where m[temp] represents the slope of the laser diode current versusphotodiode response at a particular temperature and b[temp] representsthe y-intercept (or offset value) of the laser diode current versusphotodiode response at a particular temperature. As the temperature ofthe laser diode changes, the values of m[temp] and b[temp] also change.As such, a relationship or table including values of m[temp] and b[temp]at particular photodiode voltages is stored in memory, such as in buffer106 in FIG. 1. In other embodiments, where the relationship is morecomplex, curve fitting can be used.

FIG. 5 illustrates a graphical representation 340 illustrating therelationship of optimal applied laser current (I_(op)) of the laserdiode versus the temperature of the data storage device for several datastorage devices measured during the engineering phase. Theserelationships may vary by data track radius and/or recording zone. Anexemplary method in determining I_(op) includes a triple-track (withsqueeze) bit error rate measurement and optimization of laser currentfor maximum bit error rate at each temperature point. In someembodiments, laser current can be optimized for optimal areal density.In other embodiments, laser current can be optimized for optimalsequential data rate performance. In still other embodiments, lasercurrent can be optimized to maximize the reliability of data storagedevice 100.

For data storage device 100 illustrated in FIG. 1, a curve is fitted tothe data and the following equation can be used to model therelationship:I _(op)(Temp)=(I _(op) _(@20° C.) )+0.03e ^((0.1×Temp))  (Eqn. 3)

In addition to characterizing the I_(op), it is also useful to determinethe sensitivity of bit error rate to I_(op) variations. Specifically, itis important to study how much change in I_(op) from the ideal is nearlyinconsequential, and how much change in I_(op) is tolerable before harderrors are induced on the data track of interest or the adjacent datatracks. This information is used to set limits, which will be discussedbelow. During device manufacturing, it is often only practical (orcost-effective) to measure the optimal applied laser current (I_(op)) ata few temperature points (e.g. at one or two temperatures). If I_(op) ismeasured at a specific temperature, we can use Eqn. 3 or a similar curvefit to compute the I_(op) for all temperatures.

To use sensed laser output power to set the applied laser current to alaser diode in a HAMR device, many devices are first characterizedduring the engineering phase of product development using the generalforms of Eqns. 1-3. Then, during manufacturing, the optimal lasercurrent (I_(op)) for each recording zone at one or more temperatures isdetermined and the relationship between I_(op) and sensed laser power attwo or more temperatures and two or more laser currents is determined.Eqns. 1 and 2 are used to extrapolate the relationship to temperaturenot measured if the relationship is linear. These determinations areused for setting the applied laser current to the laser diode duringnormal data storage device operation.

FIG. 6 is a block diagram 450 illustrating a method of calibrating (orsetting) laser diode current in a HAMR device during normal deviceoperation. In one embodiment, the calibration occurs when laser assembly126 is active (i.e., when laser assembly 126 is supplied with sufficientcurrent for writing or erasing data to medium 108). In anotherembodiment, the calibration occurs when laser assembly 126 is biased(i.e., when laser assembly 126 is supplied with insufficient current forwriting or erasing data). Use of calibration in the latter embodimentmight be useful over the calibration in the former embodiment if noisefrom the write process is excessive.

At block 452, the data storage device temperature is measured. Forexample, data storage device temperature can be sensed by thermistor128, which is located in proximity to HAMR device 118. At block 454,laser output power or energy of a laser diode located in laser assembly126 is measured. For example, laser output power can be sensed byphotodetector 127 or bolometer 131.

At block 456, power error is determined using the following equation:PowerError=V _(measured) −V _(op)  (Eqn. 4)where V_(measured) represents the measured voltage of the laser outputpower and V_(op) represents the optimal, target or desired voltage oflaser output power. The power error (PowerError) is the differencebetween the sensed laser output power and the optimal laser outputpower. The magnitude of the power error (|PowerError|) is the absolutevalue of the difference between the sensed laser output power and theoptimal laser output power. The optimal laser output power is typicallya power where the medium has minimal bit error rate (BER) and adjacenttrack erasure (ATE).

At block 458, the magnitude of power error is compared to a firstthreshold value (T₁) to determine whether the current being applied tothe laser or laser diode needs to be adjusted. The first threshold (T₁)is related to how great the power error can be while maintaining theintegrity of data on medium 108. If the power error is less than thefirst threshold value (T₁), then the calibration ends. If the powererror is greater than the first threshold value (T₁), the calibrationproceeds to block 460.

At block 460, the magnitude of power error is compare to a secondthreshold value (T₂) to determine whether the current being applied tothe laser or laser diode is to be adjusted. The second threshold (T₂) isrelated to how great the power error can be while still maintaining theintegrity of data on medium 108. If the power error is less than thesecond threshold value (T₂), then the calibration proceeds to block 462.If the power error is greater than the second threshold value (T₂), thecalibration proceeds to block 464.

Blocks 464, 466 and 468 pertain to the adjustment of the applied laserinput current to the laser. This adjustment is made as a function of thepower error and the measured temperature when the power error is greaterthan the second threshold (T₂). At block 462, the following equation isused to determine the laser current adjustment:

$\begin{matrix}{{LaserCurrentAdjustment} = \frac{- {PowerError}}{m\lbrack{temp}\rbrack}} & \left( {{Eqn}.\mspace{14mu} 5} \right)\end{matrix}$where PowerError was determined above in regards to block 456 using Eqn.4 and m[temp] is the slope at the measured temperature. In other words,m[temp] is accessed from the relationship of values stored in memory,such as in buffer 106, as previously discussed. In particular, therelationship includes values of slope and y-intercept that characterizethe relationship amongst temperature, applied laser input current andlaser output power.

Block 466 is an optional step in the calibration illustrated in FIG. 6.At block 466, the LaserCurrentAdjustment determined by Eqn. 5 can bestored in memory, such as non-volatile memory, so that it can be used asa starting point for future calibrations if device power is lost. Atblock 468, the LaserCurrentAdjustment determined in block 456 is appliedor added to the programmed current on future operations. Morespecifically, the new laser current needed can be written to a registerin preamp 107.

In one embodiment, the adjustments can be made globally (i.e., oneadjustment value per head or transducer) or more locally (i.e., per headand/or per zone). If correction is performed globally, the amplitude ofcorrection per zone can be pro-rated depending on ideal laser current inthe zone. If correction adjustments are fine-tuned per zone, a largedisruption in one zone can trigger similar adjustments in other zones,or can trigger re-calibration activities before writing user data.

After the laser current adjustment has been made, the calibration endsor, in the alternative, steps 454, 456, 458 and 460 can be repeateduntil the difference between the sensed laser output power and theoptimal laser output power is below the first threshold (T₁). In oneembodiment and where the power error is greater than the secondthreshold value (T₂), the calibration can optionally pass to block 464and perform a re-characterization routine as will be discussed below andillustrated in block diagram 570 of FIG. 7 or the calibration can passto block 469 and report a hardware error. After either there-characterization routine is completed or the hardware error isreported, the calibration can end or, in the alternative, steps 454,456, 458 and 460 can be repeated until the difference between the sensedlaser output power and the optimal laser output power is below the firstthreshold value (T₁).

Block diagram 570 in FIG. 7 illustrates a characterization orre-characterization routine or method that can be performed when themagnitude of power error is too large or under other circumstances orfor other reasons, such as host initiated re-characterization,time-based re-characterization or based on a fixed type of driveactivity. The method performed in FIG. 7 characterizes orre-characterizes the relationship amongst temperature, applied lasercurrent and laser output power. If the calibration illustrated in FIG. 6sends the process to this re-characterization routine, this is an alertthat something might have drastically changed in the recording head orelsewhere in the system. If this occurs, it is important to have thedevice perform additional internal assessments to determine whether itis still operating properly and functional enough to continue.

At block 572, control circuitry 102 (FIG. 1) performs a seek to moveslider 118 to seek to a reserve track on medium 108. For example, thereserved track is the middle track in a band of five tracks and isreserved for testing such that if we erase this track no user data isdamaged. At block 573, a first laser current (I₁) is applied. At block574, a first laser output power (P₁) is measured based on the appliedfirst laser current (I₁). At block 575, a second laser current (I₂) thatis different from the first laser current (I₁) is applied. At block 576,a second laser output power (P₂) is measured based on the applied secondlaser current (I₂).

At block 577, the relationship determined above is adjusted using thefirst laser current (I₁) applied in block 573, the first laser outputpower (P₁) measured in block 574, the second laser current (I₂) appliedin block 575 and the second laser output power (P₂) measured in block576. For example, if the relationship is linear, Eqns. 1-2 can be usedto compute the slope (m) and y-intercept or offset values (b). The newcomputed slope and offset values replace the old values stored in thetable and therefore adjust the relationship.

At block 578, the laser current is set to the optimal laser current(I_(op)) as was determined during manufacturing. With a change in lasercurrent, at block 579, the fly height of recording head 118 is alsoadjusted. After I_(op) is set, a metric of recording performance ismeasured to determine if the new relationship is acceptable. Therecording performance metric can be, but is not limited to, trackaverage amplitude (TAA), track width, ability to erase known informationand bit error rate. In FIG. 7 and in one embodiment, block diagram 570uses bit error rate (BER) as the recording performance metric.

In this embodiment, at block 580 bit error rate (BER) with squeeze ismeasured. BER with squeeze means that a data track is first written andthen adjacent tracks are written on either side of the data track at afixed track pitch. The track pitch of the adjacent tracks relative tothe data track is directly proportional to the allowable amount ofencroachment of the adjacent tracks on the data track. At block 581, itis determined whether the BER of a track written with squeeze isacceptable. If so, then the method returns to block 464 in block diagram450. If not, then the method passes to block 582 to measure an isolatedtrack BER, which means only a single data track is written with noadjacent tracks.

At block 583, it is determined whether the BER of the isolated datatrack written is acceptable. If so, then the method passes to block 584and a determination is made as to whether the number (n) of BERmeasurements or the number of iterations (n) of BER measurements made inthe re-characterization routine are greater than or less than anacceptable or threshold amount of iterations. If the iteration number(n) is less than a threshold number of iterations, the method passes toblock 585 and laser power output is reduced. If the iteration number (n)is greater than the threshold number of iterations, the method returnsto block 464 in block diagram 450.

If the BER of the isolated data track is unacceptable, then the methodpasses to block 586 and a determination is made as to whether the number(n) of BER measurements or the number of iterations (n) of BERmeasurements made in the re-characterization routine are greater than orless than an acceptable or threshold amount of iterations. If theiteration number (n) is less than a threshold number of iterations, themethod passes to block 587 and laser power output is increased. If theiteration number (n) is greater than the threshold number of iterations,the method returns to block 464 in block diagram 450.

After either laser power is reduced in block 585 or increased in block587, the fly height of recording head 118 is adjusted, the number ofiterations is increased by one and the method returns to block 580 toagain measure BER with squeeze. The method repeats steps 580, 581, 582and 583 until the BER is a satisfactory value. If an acceptable BER isnever achieved within a set number of iterations, the recording head isbad and data storage device 100 should report this status to the host114.

The calibration method and routine described in FIG. 6 can be calledduring various data storage device operations. In one embodiment, thecalibration can be called when laser assembly 126 is in an active statewhere the laser power is sufficient to write and erase data, such asduring normal write operation. In another embodiment, the calibrationcan be called when laser assembly 126 is in a biased state where thelaser power is insufficient to write and erase data, such as during awrite seek.

In the exemplary write seek embodiment, the calibration can be called ona write seek for each servo wedge ahead of a write operation. In thisway, before data is written, any temperature and other effects of thelaser are compensated. This will take into account and correct not onlyfor drive temperature changes but also changes in laser due toself-heating. In addition, if the output of preamp 107 also changes withtemperature, using the calibration routine on each wedge will alsocorrect for these effects. In order for the calibration routine to workon each servo wedge, it must be completed during the servo gate period.If this is not possible, the calibration routine can update one wedgelate. For example, on wedge 1 values are measured, but they are notupdated until wedge 2.

In the exemplary write seek embodiment, the calibration canalternatively be called by the firmware during write seek. When thewrite seek command is issued, the head is moved to the desired track.Since the laser output power during seeking is lower than what isrequired to write on medium 108, servo information will not be damaged.In this embodiment, the temperature effects are corrected on every writecommand from control circuitry 102. For long sequential writes, thecalibration routine could be performed during track switch seeks orduring head switches. For very long sequential writes, writing could beinterrupted to issue the calibration routine.

Other opportunities for the calibration routine to be called when laserassembly 126 is in either active or biased states include on a headswitch in a multi-headed device, on a temperature change as measured byan onboard temperature sensor in the date storage device, on atemperature change of preamp 107 or channel 110, on a write and verifycommand as part of a detected error metric exceeding a threshold, on afixed interval (i.e., time, quantity of sectors written, etc.) asspecified by firmware, when the data storage device exits from an idlecondition, on a reading from an internal bolometer in the head, such asa fly-height thermistor sensor or other thermal resistor, on thedetection of variance of the laser diode forward voltage drop in laserassembly 126 and during drive manufacturing, either at one or twotemperatures. If only performed at one temperature during manufacture,the compensation for temperature uses fixed default constants.

FIG. 8 is a block diagram 650 illustrating a method of calibrating (orsetting) laser diode current in a HAMR device during normal deviceoperation. In one embodiment, the calibration occurs when laser assembly126 is active. In another embodiment, the calibration occurs when laserassembly 126 is biased. In FIG. 8, rather than using control circuitryin data storage device 100, hardware in preamp 107, such as compensationcircuitry 109, is used to make adjustments to laser input current toachieve optimal laser output power. For example, Texas Instruments®TI5563 preamp contains such hardware compensation circuitry. Datastorage device 100 need only control preamp registers to invoke preampinternal hardware for compensation.

At block 652, data storage device 100 measures the data storage devicetemperature. For example, data storage device temperature can be sensedby thermistor 128, which is located in proximity to HAMR device 118. Atblock 654, data storage device 100 measures laser output power or energyof a laser diode located in laser assembly 126. For example, laseroutput power can be sensed by a photodetector located in laser assembly126 or bolometer 131 located in optical wave guide 119. At block 656,data storage device 100 supplies the measured laser output power tocompensation circuitry 109 in preamp 107. At block 658, controlcircuitry 102 supplies an optimal laser output power based on themeasured temperature to compensation circuitry 109. The optimal laseroutput power is accessed from the relationship stored in memory, such asbuffer 106, and as previously discussed. In particular, if therelationship is linear buffer 106 can include values of slope andy-intercept that characterize the linear relationship amongsttemperature, applied laser input current and laser output power.

At block 660, control circuitry 102 determines if a notification wasreceived from preamp 107 that the adjustment needed to the laser inputcurrent is greater than a threshold (T) amount. If so, the method passesto block 664 and performs a re-characterization routine, which wasdiscussed and illustrated in block diagram 570 of FIG. 7, or thecalibration can pass to block 669 and report a hardware error. Aftereither the re-characterization routine is completed or the hardwareerror is reported, the calibration ends. If no notification is receivedfrom preamp 107, the calibration ends.

Additions to the calibrations illustrated in FIGS. 6 and 8 includeperforming fly height adjustments to slider 122 based on the adjustmentsto laser output power caused by temperature.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A method comprising: applying at least one laserinput current to a laser in a heat assisted magnetic recording device;measuring laser output power of the laser at the at least one appliedlaser current; characterizing a relationship amongst temperature, theapplied laser input current and the laser output power; setting thelaser input current to an optimal laser current as determined atmanufacturing; and measuring a metric of recording performance todetermine if the relationship is acceptable.
 2. The method of claim 1,wherein applying the at least one laser current to the laser andmeasuring the laser output power of the laser comprises applying the atleast one laser current to the laser when the heat assisted magneticrecording device is positioned over a reserved test track.
 3. The methodof claim 1, wherein applying the at least one laser current to the laserand measuring the laser output power of the laser comprises: applying afirst laser input current to the laser; measuring a first laser outputpower of the laser at the first laser input current; applying a secondlaser input current that is different from the first laser input currentto the laser; and measuring a second laser output power of the laser atthe second laser input current.
 4. The method of claim 1, whereincharacterizing the relationship amongst temperature, the applied laserinput current and the laser output power comprises characterizing alinear relationship amongst the temperature, the applied laser inputcurrent and the laser output power by computing a slope and ay-intercept based on the at least one laser current applied to the laserand the measured laser output power.
 5. The method of claim 1, furthercomprising adjusting a fly height of a slider of the heat assistedmagnetic recording device after setting the laser current to the optimallaser current as determined at manufacturing.
 6. The method of claim 1,wherein measuring the metric of recording performance to determine ifthe relationship is acceptable comprises measuring one of track averageamplitude, track width, an ability to erase known information andbit-error rate.
 7. The method of claim 1, wherein measuring the metricof recording performance to determine if the relationship is acceptablecomprises: measuring bit-error rate with squeeze; and determining if thebit-error rate with squeeze is acceptable.
 8. The method of claim 7,further comprising: measuring bit-error rate on an isolated data trackif the measured bit-error rate with squeeze is unacceptable; anddetermining if the bit-error rate on the isolated data track isacceptable.
 9. The method of claim 8, further comprising: reducing thelaser output power of the laser if the measured bit-error rate on theisolated data track is acceptable; and repeating the step of measuringbit-error rate with squeeze if the number of times the bit-error ratehas been measured has not exceeded a threshold value and until thebit-error with squeeze is acceptable.
 10. The method of claim 9, furthercomprising adjusting fly height of a slider of the heat assistedmagnetic recording device after reducing the laser output power of thelaser.
 11. The method of claim 8, further comprising: increasing thelaser output power of the laser if the measured bit-error rate on theisolated data track is unacceptable; and repeating the step of measuringbit-error rate with squeeze if the number of times the bit-error ratehas been measured has not exceeded a threshold value and until thebit-error rate with squeeze is acceptable.
 12. The method of claim 11,further comprising adjusting fly height of a slider of the heat assistedmagnetic recording device after increasing the laser output power of thelaser.
 13. A data storage device comprising: a medium; a heat assistedmagnetic recording device including a laser for heating the medium whilewriting data; and control circuitry configured to: apply at least onelaser current to a laser in the heat assisted magnetic recording device;measure laser output power of the laser at the at least one appliedlaser current; characterize a relationship amongst temperature, theapplied laser input current and the laser output power; and measure ametric of recording performance to determine if the relationship isacceptable.
 14. The data storage device of claim 13, wherein when thecontrol circuitry applies the at least one laser current to the laserand measures the laser output power of the laser, the control circuitryis configured to: apply a first laser input current to the laser;measure a first laser output power of the laser at the first laser inputcurrent; apply a second laser input current that is different from thefirst laser input current to the laser; and measure a second laseroutput power of the laser at the second laser input current.
 15. Thedata storage device of claim 13, wherein when the control circuitrycharacterizes the relationship amongst the temperature, the appliedlaser input current and the laser output power, the control circuitry isconfigured to characterize a linear relationship amongst thetemperature, the applied laser input current and the laser output powerby computing a slope and a y-intercept based on the at least one lasercurrent applied to the laser and the measured laser output power. 16.The data storage device of claim 13, wherein the control circuitry isfurther configured to adjust a fly height of a slider of the heatassisted magnetic recording device after setting the laser current to anoptimal laser current as determined at manufacturing.
 17. A methodcomprising: measuring a first laser output power of a laser used torecord data on a head assisted magnetic recording medium, the firstlaser output power is measured at a first laser input current; measuringa second laser output power of the laser, the second laser output poweris measured at a second laser input that is greater than or less thanthe first laser input current; characterizing a relationship amongsttemperature, the laser input current and the laser output power; andverifying the relationship by measuring a metric of recordingperformance.
 18. The method of claim 17, wherein the first laser inputcurrent and the second laser input current comprise currents that areinsufficient to record or erase data on the recording medium.
 19. Themethod of claim 17, wherein the first laser input current and the secondlaser input current comprise currents that are sufficient to record orerase data on the recording medium.
 20. The method of claim 19, whereincharacterizing the relationship amongst the temperature, the appliedlaser input current and the laser output power comprises characterizinga linear relationship amongst the temperature, the applied laser inputcurrent and the laser output power by computing a slope and ay-intercept based on the first laser current applied to the laser, thefirst measured laser output power, the second laser current applied tothe laser and the second measured laser output power.